Condenser Vacuum Loss with 40 Days Left to the Overhaul
A 210 MW unit is running at 190 MW with both circulating water pumps in service. The condenser vacuum is 682 mm Hg against a design of 710 mm Hg at 28 °C CW inlet. Today's CW inlet is 33 °C and outlet 41 °C. The station's correction curve gives 3 mm Hg per °C of CW inlet, and 4.5 kcal/kWh of heat rate per mm Hg of vacuum. Air-ejector suction temperature is normal, but a vacuum-tightness test (ejector isolated) shows a decay of 4 mm Hg/min against an acceptance limit of 1 mm Hg/min. Condensate dissolved oxygen is 85 µg/l against a design of 20 µg/l, and condensate subcooling is 3.8 °C against 1.0 °C. Hotwell level is steady and there is no tube leak (cation conductivity 0.15 µS/cm). Coal GCV is 3,800 kcal/kg at ₹2,800/t. The condenser was last tube-cleaned 26 months ago, and the terminal difference is 7.6 °C against a design of 3.2 °C. Forty days remain before the planned overhaul and the unit is not restricted by the grid.
1. Separate the effect of the high CW inlet temperature from the station's own deficiencies, and quantify the heat-rate and coal-cost penalty attributable to the plant.
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Step 1 - correct the vacuum to design CW inlet. The inlet is 5 °C above design, and at 3 mm Hg per °C that explains 15 mm Hg of the shortfall: 682 + 15 = 697 mm Hg corrected, against a design of 710 mm Hg. The residual shortfall attributable to the plant is therefore 13 mm Hg, not 28.
Step 2 - convert to heat rate. 13 × 4.5 = 58.5 kcal/kWh.
Step 3 - convert to coal. 58.5 kcal/kWh × 190,000 kW = 11.12 Gcal/h = 11.12 × 10^6 kcal/h. Divided by 3,800 kcal/kg = 2,926 kg/h ≈ 2.9 t/h of extra coal.
Step 4 - cost. 2.93 t/h × ₹2,800/t ≈ ₹8,200 per hour, i.e. about ₹1.97 lakh per day and roughly ₹59 lakh over the 30 days before overhaul.
The high inlet temperature is a site/seasonal condition and cannot be corrected, but it must not be allowed to mask the plant's own 13 mm Hg loss - the correction is what makes the penalty figure defensible before the management or the regulator.
2. Rank the corrective actions in execution order and state what will be done in the next 40 days versus at the overhaul.
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Ranking by cause, evidence and cost:
Air ingress - first priority. Evidence: vacuum decay of 4 mm Hg/min against a 1 mm Hg/min limit, DO 85 µg/l against 20, and subcooling 3.8 °C against 1.0. Normal ejector suction temperature does not exclude ingress, because it responds slowly. Leak-hunt at turbine glands, low-pressure bypass, flash box, ejector nozzle and hogging connection, rupture disc and manhole gaskets. This needs no outage and gives the largest immediate gain in DO and corrosion life.
Tube fouling / water-box debris - second priority. Evidence: terminal difference 7.6 °C against 3.2 °C. It needs the condenser to be opened, so it belongs to the 40-day plan, not to today.
CW flow - not a problem. At 190 MW the CW temperature rise is 8 °C against a design of 9 °C, so the flow is slightly more than design; there is no flow shortage to fix.
Next 40 days: eliminate air ingress, watch exhaust-hood temperature and back pressure daily, and back the unit down only if the hood temperature approaches the turbine alarm limit. At overhaul: mechanical/chemical tube cleaning, water-box inspection, ejector nozzle and gland overhaul, and a repeat tightness test before box-up.
3. The shift in-charge wants to stop one CW pump, arguing that the low CW outlet temperature is over-cooling the condensate and causing the subcooling. Evaluate the decision.
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Reject it. Vacuum is fixed by the saturation temperature of the condenser, which falls as the heat-transfer rate rises - within the operating range, more CW flow means a lower back pressure, not a higher one. Stopping a pump raises the CW outlet temperature and the back pressure, and adds to the heat-rate penalty already being carried. The saving in auxiliary power is small compared with the turbine heat-rate loss, and the unit is not grid-restricted, so there is no system reason to shed CW pump load.
The assumed link between low CW outlet temperature and 3.8 °C subcooling is also wrong in direction. Subcooling of that magnitude with high DO and a 4 mm Hg/min decay points to air blanketing the tubes and raising the partial pressure of air in the shell, not to the condensate being over-cooled by cold tubes. The correct response is the leak hunt, not a change in CW flow.
A pump-optimisation study is legitimate only when the temperature rise is far below design at full load and the back pressure is already better than the corrected design value - neither is true here.